Speed control device for ac electric motor

ABSTRACT

A speed control apparatus of an AC motor according to the invention has current controllers for performing proportional integration control of an excitation component current and a torque component current of two components on rotating Cartesian two-axis coordinates into which a current of the AC motor is separated. The speed control apparatus comprises a first subtracter  1  for finding torque component voltage saturation amount ΔV q  from torque component voltage component V q ′ output from a torque component current controller  47   a  and torque component voltage command V q * output from a torque component voltage limiter  54   a,  a first integrator  2  for holding the torque component voltage saturation amount ΔV q , a magnetic flux command corrector  3   a  for outputting magnetic flux command correction amount Δφ 2d  from the held torque component voltage saturation amount ΔV q ′ and rotation angular speed ω of Cartesian two-axis coordinates, and a second subtracter  4  for subtracting the magnetic flux command correction amount Δφ 2d  from magnetic flux command φ 2d * and outputting magnetic flux correction command φ 2d * cmd .

TECHNICAL FIELD

[0001] This invention relates to a speed control apparatus of an AC motor and in particular to improvement in characteristic in a higher-speed area than rated speed.

BACKGROUND OF THE INVENTION

[0002] In current control of an AC motor, often vector control is performed wherein the current of the AC motor is disassembled into an excitation component (which will be hereinafter referred to as d axis) and a torque component (which will be hereinafter referred to as q axis), of components on rotating Cartesian two-axis coordinates (which will be hereinafter referred to as dq-axis coordinates) and the components are controlled separately. The case of an induction motor will be discussed below as a related art.

[0003]FIG. 16 is a drawing to show the configuration of a speed control apparatus of an induction motor in a related art. In the figure, numeral 31 denotes an induction motor, numeral 32 denotes a PWM inverter for supplying electric power to the induction motor 31 based on voltage command Vu*, Vv*, Vw* described later, numerals 33 a, 33 b, and 33 c denote current detectors for detecting currents i_(u), i_(v), and i_(w) of the induction motor 31, and numeral 34 denotes a speed detector for detecting rotation speed ω_(r) of the induction motor 31. Numeral 35 denotes a secondary magnetic flux calculator for calculating magnetic flux φ_(2d) based on d-axis current i_(ld) described later, numeral 36 denotes a slip frequency calculator for calculating slip angular frequency ω_(s) based on q-axis current i_(lq) described later and the magnetic flux φ_(2d), numeral 37 denotes a coordinate rotation angular speed calculator for calculating rotation angular speed ω of dq-axis coordinates based on the slip angular frequency ω_(s) calculated by the slip frequency calculator 36 and the rotation speed ω_(r) of the induction motor 31 detected by the speed detector 34, and numeral 38 denotes an integrator for integrating the rotation angular speed ω and outputting phase angle θ of dq-axis coordinates. Numeral 39 denotes a three-phase to two-phase coordinate converter for disassembling the currents i_(u), i_(v), and i_(w) of the current detectors 33 a, 33 b, and 33 c into the d-axis current i_(ld) and the q-axis current i_(lq) on the dq-axis coordinates based on the phase angle θ of the dq-axis coordinates and outputting the d-axis current i_(ld) and the q-axis current i_(lq).

[0004] Numeral 40 denotes a subtracter for outputting magnetic flux deviation e_(f) between magnetic flux command φ_(2d)* and the magnetic flux φ_(2d) output by the secondary magnetic flux calculator 35, numeral 41 denotes a magnetic flux controller for controlling proportional integration (which will be hereinafter referred to as PI) so that the magnetic flux deviation e_(f) becomes 0 and outputting d-axis current component i_(ld)′, numeral 42 denotes a subtracter for outputting speed deviation e_(w) between speed command ω_(r)* and the rotation speed ω_(r) of the induction motor 31 output by the speed detector 34, and numeral 43 denotes a speed controller for controlling PI so that the speed deviation e_(w) becomes 0 and outputting q-axis current component i_(lq)′.

[0005] Numeral 44 denotes a subtracter for outputting current deviation e_(id) between d-axis current command i_(ld)* and d-axis current i_(ld), numeral 45 b denotes a d-axis current controller for controlling PI so that the current deviation e_(id) becomes 0 and outputting d-axis voltage component V_(d)′, numeral 46 denotes a subtracter for outputting current deviation e_(iq) between q-axis current command i_(lq)* and q-axis current i_(lq), numeral 47 b denotes a q-axis current controller for controlling PI so that the current deviation e_(iq) becomes 0 and outputting q-axis voltage component V_(q)′, and numeral 48 denotes a two-phase to three-phase coordinate converter for converting d-axis voltage command V_(d)* and q-axis voltage command V_(q)* into the voltage commands Vu*, Vv*, and Vw* on three-phase AC coordinates based on the phase angle θ of the dq-axis coordinates and outputting the voltage commands as voltage commands of the PWM inverter 32.

[0006] Numeral 51 denotes a d-axis current limiter for limiting the d-axis current component i_(ld)′ within a predetermined range and outputting the d-axis current command i_(ld)*, and numeral 52 denotes a q-axis current limiter for limiting the q-axis current component i_(lq)′ within a predetermined range and outputting the q-axis current command i_(lq)*. Numeral 53 b denotes a d-axis voltage limiter for limiting the d-axis voltage component V_(d)′ within a predetermined range and outputting the d-axis voltage command V_(d)*, and numeral 54 b denotes a q-axis voltage limiter for limiting the q-axis voltage component V_(q)′ within a predetermined range and outputting the q-axis voltage command Vq*.

[0007] Numeral 55 denotes a magnetic flux command generation section for arbitrarily giving the magnetic flux command φ_(2d)* of the induction motor. The speed command ω_(r)* is given arbitrarily from the outside.

[0008]FIG. 17 is a drawing to show the configuration of the PI controller of the magnetic flux controller 41, the speed controller 43, the d-axis current controller 45 b, the q-axis current controller 47 b, etc., in FIG. 16. In FIG. 17, numeral 61 denotes a coefficient unit corresponding to proportional gain K_(P) of the PI controller, numeral 62 denotes a coefficient unit corresponding to integration gain K_(I) of the PI controller, numeral 63 b denotes an integrator having a function of stopping calculation, and numeral 64 denotes an adder for adding the proportional component and the integration component.

[0009] Letter e denotes deviation input to the PI controller and U′ denotes control input output from the PI controller. As for the magnetic flux controller 41, e corresponds to the magnetic flux deviation e_(f) between the magnetic flux command φ_(2d)* and the magnetic flux φ_(2d) output by the secondary magnetic flux calculator 35, and U′ corresponds to the d-axis current component i_(ld)′. As for the speed controller 43, e corresponds to the speed deviation e_(w) between the speed command ω_(r)* and the rotation speed ω_(r) of the induction motor 31 output by the speed detector 34, and U′ corresponds to the q-axis current component i_(lq)′. As for the d-axis current controller 45 b, e corresponds to the current deviation e_(id) between the d-axis current command i_(ld)* and the d-axis current i_(ld), and U′ corresponds to the d-axis voltage component V_(d)′. As for the q-axis current controller 47 b, e corresponds to the current deviation e_(iq) between the q-axis current command i_(lq)* and the q-axis current i_(lq), and U′ corresponds to the q-axis voltage component V_(q)′.

[0010] The basic operation of the vector control in the induction motor will be discussed with FIGS. 16 and 17.

[0011] As shown in FIG. 16, the vector control is implemented using a plurality of PI controllers of the magnetic flux controller 41, the speed controller 43, the d-axis current controller 45 b, the q-axis current controller 47 b, etc., in combination.

[0012] The subtracter at the stage preceding each PI controller (subtracter 40, subtracter 42, subtracter 44, subtracter 46) outputs deviation e (e_(f), e_(ω), e_(id), e_(iq)) from the command value and actually detected value.

[0013] The PI controller is a controller for setting the deviation output from the subtracter at the preceding stage to 0 (matching the command value and actually detected value with each other). Each PI controller inputs the deviation e output from the subtracter at the preceding stage and outputs such control input U′ (i_(ld)′, i_(lq)′, V_(d)′, V_(q)′) setting the deviation e to 0 based on the following expression (1):

U′=(K _(P)+(K _(I) /s))·e  (1)

[0014] The block diagram of expression (1) is shown in FIG. 17 wherein K_(P) denotes the proportional gain of the PI controller and K_(I) denotes the integration gain of the PI controller. The PI controller used in FIG. 16 (magnetic flux controller 41, speed controller 43, d-axis current controller 45 b, q-axis current controller 47 b) is the PI controller shown in FIG. 17, but the PI controllers differ, in values of K_(P) and K_(I).

[0015] In the magnetic flux controller 41 or the speed controller 43, the d-axis current component i_(ld)′ or the q-axis current component i_(lq)′ corresponds to the control input U′, but cannot be set to a value equal to or greater than maximum output current value i_(max) allowed by the PWM inverter 32. Then, the d-axis current limiter 51, the q-axis current limiter 52 limits so that the control input U′ output from the magnetic flux controller 41, the speed controller 43 (d-axis current component i_(ld)′, q-axis current component i_(lq)′) does not exceed the maximum output current value i_(max) allowed by the PWM inverter 32.

[0016] In the d-axis current controller 45 b or the q-axis current controller 47 b, the d-axis voltage component V_(d)′ or the q-axis voltage component V_(q)′ corresponds to the control input U′, but cannot be set to a value equal to or greater than bus voltage V_(DC) of the PWM inverter 32. Thus, the d-axis voltage limiter 53 b, the q-axis voltage limiter 54 b limits so that the control input U′ output from the d-axis current controller 45 b or the q-axis current controller 47 b (d-axis voltage component V_(d)′ or q-axis voltage component V_(q)′) does not exceed the bus voltage V_(DC) of the PWM inverter 32.

[0017] However, the limit values of the d-axis current limiter 51, the q-axis current limiter 52, the d-axis voltage limiter 53 b, and the q-axis voltage limiter 54 b need not necessarily be the same.

[0018] As described above, in the speed control apparatus of the induction motor in the related art, the limiters 51, 52, 53 b, and 54 b are provided for outputs of the PI controllers 41, 43, 45 b, and 47 b and if the control input U′ is limited by the limiter 51, 52, 53 b, 54 b, input deviation e does not become 0 for ever and if the deviation e continues to be accumulated in the integrator 63 b in the PI controller, a phenomenon called control input saturation arises, causing a vibratory output response called overshoot or hunting; this is a problem.

[0019] Thus, if the control input U′ exceeds the limit value of the limiter 51, 52, 53 b, 54 b, empirically the integration operation of the integrator 63 b in the PI controller is stopped, thereby avoiding continuing to accumulate the deviation e for eliminating control input saturation, thereby obtaining a stable response.

[0020]FIG. 18 is a graph plotting the d-axis voltage component V_(d)′ and the q-axis voltage component V_(q)′ based on expressions for finding terminal-to-terminal voltage in a stationary state in the induction motor described later. In the figure, (a), (c), and (e) indicate the d-axis voltage component V_(d)′ and (b), (d), and (f) indicate the q-axis voltage component V_(q)′.

[0021]FIG. 19 is a graph to show the limit values of the q-axis current limiter relative to the rotation speed ω_(r).

[0022]FIG. 20 is a graph to show the maximum allowable values of the magnetic flux command φ_(2d)* that can be arbitrarily output from the magnetic flux command generation section relative to the rotation speed ω_(r).

[0023]FIG. 18 corresponds to FIGS. 19 and 20. If the limit value is changed to (a), (c), and (e) in FIG. 19, the graph of FIG. 18 becomes as (a), (c), and (e). If the maximum allowable value is changed to (b), (d), and (f) in FIG. 20, the graph of FIG. 18 becomes as (b), (d), and (f).

[0024] To operate the induction motor at rated speed or more, the d-axis voltage component V_(d)′ and the q-axis voltage component V_(q)′ output from the d-axis current controller 45 b and the q-axis current controller 47 b continue to exceed the limit values of the d-axis voltage limiter 53 b and the q-axis voltage limiter 54 b stationarily. The above-described method of stopping the integration operation if the control input exceeds the limit value is means for temporarily avoiding the uncontrollable state of control input saturation and is effective for transient control input saturation, but cannot be used if control input saturation continues to occur stationarily as when the induction motor is operated at the rated speed or more.

[0025] A method in related art for eliminating control input saturation of the voltage components V_(d)′ and V_(q)′ occurring stationarily at the rated speed or more will be discussed with FIGS. 18 to 20. Such control input saturation of V_(d)′ and V_(q)′ in high-speed area is particularly called voltage saturation.

[0026] As for the induction motor, the d-axis voltage component V_(d)′ and the q-axis voltage component V_(q)′ in a stationary state are given according to the following expressions (2) and (3):

V _(d) ′=R ₁ ·i _(1d) −L ₁ ·σ·ω·i _(lq)  (2)

V _(q) ′=R ₁ ·i _(1q)+(L ₁ /M)·ω·φ_(2d)  (3)

[0027] where R_(l) denotes primary resistance of the induction motor 31, L₁ denotes primary side self inductance, M denotes mutual inductance, and σ denotes a leakage coefficient.

[0028] To operate the induction motor at the rated speed or more, the second term component in expression (2), (3) becomes very larger than the first term component and thus expressions (2) and (3) can be approximated by the following expressions (4) and (5):

V _(d) ′=−L ₁ ·σ·ω·i _(lq)  (4)

V _(q)′=(L ₁ /M)·ω·φ_(2d)  (5)

[0029] The q-axis current limiter 52 is a fixed limiter and the q-axis current limiter value is indicated by FIG. 19 (a). Here, assuming that the q-axis current i_(lq) flows as much as the limit value, V_(d)′ becomes the graph of FIG. 18 (a) according to expression (4). The maximum allowable value of φ_(2d)* that can be arbitrarily output from the magnetic flux command generation section 55 is indicated by FIG. 20 (b). Here, assuming that the magnetic flux φ_(2d) takes the same value as the maximum allowable value, V_(q)′ becomes the graph of FIG. 18 (b) according to expression (5).

[0030] From FIGS. 18 (a) and (b), it is seen that to operate the induction motor at rotation speed ω_(base) or more, the voltage component V_(q)′ becomes saturated exceeding the output limit value of the PWM inverter 32 ±V_(max) and that to operate the induction motor at rotation speed ω_(base2) or more, both the voltage components V_(d)′ and V_(q)′ become saturated exceeding the output limit value of the PWM inverter 32 ±V_(max).

[0031] Since voltage saturation occurs stationarily in such an area at the rated speed or more, the maximum allowable value of φ_(2d)* of the magnetic flux command generation section 55 and the limit value of the q-axis current limiter 52 are changed in response to the speed. For example, if a variable limiter is adopted for changing the limit value of the q-axis current limiter in a manner inversely proportional to the speed from the rotation speed ω_(base2) at which voltage saturation of the d-axis component occurs as indicated by FIG. 19 (c), even if the q-axis current i_(lq) flows as much as the limit value, V_(d)′ becomes the graph of FIG. 18 (c) according to expression (4). If the maximum allowable value of φ_(2d)* that can be arbitrarily output from the magnetic flux command generation section 55 is limited by a function inversely proportional to the speed from the rotation speed ω_(base) at which voltage saturation of the q-axis component occurs as indicated by FIG. 20 (d), even if the magnetic flux φ_(2d) takes the same value as the maximum allowable value, V_(q)′ becomes the graph of FIG. 18 (d) according to expression (5).

[0032] As described above, the limit value of the q-axis current limiter and the maximum allowable value of φ_(2d)* are changed in response to the speed, whereby the d-axis voltage component V_(d)′ and the q-axis voltage component V_(q)′ are prevented from exceeding the output limit value of the PWM inverter 32 ±V_(max) even in an area at the rated speed or more, and occurrence of voltage saturation can be suppressed, so that a stable response can be provided.

[0033] However, if the induction motor is actually turned, the voltage component V_(d)′, V_(q)′ may become larger than FIGS. 18 (c), (d) because of fluctuation of the magnitude of load or a bus voltage, and voltage saturation occurs, resulting in an unstable response.

[0034] Then, the limit value of the q-axis current limiter and the maximum allowable value of φ_(2d)* are set further lower as in FIG. 19 (e) and FIG. 20 (f) and the voltage component V_(d)′, V_(q)′ can be provided with a margin relative to the output limit value of the PWM inverter 32 ±V_(max) as in FIGS. 18 (e), (f) for making voltage saturation hard to occur.

[0035] In this case, however, it is made impossible to make full use of the capabilities of the PWM inverter and lowering of output torque or the like is incurred; this is a problem.

[0036] To make voltage saturation hard to occur without lowering the output torque, a method of feeding back a magnetic flux command or a current command for correction if voltage saturation occurs is proposed. In this method, when voltage saturation occurs, the saturation amount is detected, an optimum correction amount to eliminate the voltage saturation is found from the saturation amount, and each command is corrected. Such feedback control is performed, whereby occurrence of voltage saturation can be suppressed and stability of control can be improved independently of the conditions of the load and the bus voltage, and it is also made possible to make full use of the capabilities of the PWM inverter.

[0037] For example, the Unexamined Japanese Patent Application No. 2000-92899 discloses a control apparatus of an induction motor, comprising a voltage saturation compensation circuit for making a comparison between a voltage command value from a current control system and a bus voltage value of a PWM inverter and integrating and if the above-mentioned bus voltage value is greater than the above-mentioned voltage command value, the voltage saturation compensation circuit for subtracting the above-mentioned integrated output from a magnetic flux command and if the above-mentioned bus voltage value is lower than the above-mentioned voltage command value, for subtracting 0 from the magnetic flux command.

[0038] In this method, the correction amount is derived in response to the voltage saturation amount and each command is corrected, so that voltage saturation can be eliminated; however, since the speed of the motor is not considered when the correction amount is determined, to cope with rapid speed change, etc., calculation of the correction amount, etc., must be thought out in such a manner that the correction amount is increased to make a prompt correction at the acceleration time and that the correction amount is suppressed to raise stability at the deceleration time, for example; the method involves such a problem.

[0039] The invention is intended for solving the problems as described above and it is an object of the invention to provide a speed control apparatus of an AC motor for making it possible to suppress occurrence of voltage saturation without performing special operation even if rapid speed change, etc., occurs.

DISCLOSURE OF THE INVENTION

[0040] According to the invention, there is provided a speed control apparatus of an AC motor having current controllers for performing proportional integration control of an excitation component current and a torque component current of two components on rotating Cartesian two-axis coordinates into which a current of the AC motor is separated, the speed control apparatus comprising: a torque component voltage limiter for limiting a torque component voltage component output from torque component current controller for performing proportional integration control of the torque component current so that the torque component voltage component becomes equal to or less than a predetermined value; a first subtracter for finding a torque component voltage saturation amount from the torque component voltage component output from the above-mentioned torque component current controller and a torque component voltage command output from the above-mentioned torque component voltage limiter; a first integrator for holding the torque component voltage saturation amount; a magnetic flux command corrector for outputting a magnetic flux command correction amount from the held torque component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a second subtracter for subtracting the magnetic flux command correction amount from a magnetic flux command and outputting a magnetic flux correction command, so that if the speed rapidly changes, etc., the optimum correction amount can always be obtained and occurrence of voltage saturation can be suppressed.

[0041] Also, there is provided a speed control apparatus of an AC motor having current controllers for performing proportional integration control of an excitation component current and a torque component current of two components on rotating Cartesian two-axis coordinates into which a current of the AC motor is separated, the speed control apparatus comprising: a torque component voltage limiter for limiting a torque component voltage component output from torque component current controller for performing proportional integration control of the torque component current so that the torque component voltage component becomes equal to or less than a predetermined value; a first subtracter for finding a torque component voltage saturation amount from the torque component voltage component output from the above mentioned torque component current controller and a torque component voltage command output from the above-mentioned torque component voltage limiter; a first integrator for holding the torque component voltage saturation amount; an excitation component current command corrector for outputting an excitation component current command correction amount from the held torque component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a third subtracter for subtracting the excitation component current command correction amount from an excitation component current command and outputting an excitation component current command correction command, so that if the speed rapidly changes, etc., the optimum correction amount can always be obtained and it is possible to suppress occurrence of voltage saturation.

[0042] Rotation speed of the above-mentioned AC motor is input to a magnetic flux command generation section for generating a magnetic flux command and a magnetic flux command is generated in response to the rotation speed of the above-mentioned AC motor, so that the magnitude of the torque component voltage saturation amount can be lessened to some extent and it is made possible to improve the stability of control of the AC motor.

[0043] Rotation speed of the above-mentioned AC motor is input to an excitation component current command generation section for generating an excitation component current command and an excitation component current command is generated in response to the rotation speed of the above-mentioned AC motor, so that the magnitude of the torque component voltage saturation amount can be lessened to some extent and it is made possible to improve the stability of control of the AC motor.

[0044] The speed control apparatus comprises an excitation component voltage limiter for limiting an excitation component voltage component output from excitation component current controller for performing proportional integration control of the excitation component current so that the excitation component voltage component becomes equal to or less than a predetermined value; a fourth subtracter for finding the excitation component voltage component output from the above-mentioned excitation component current controller and an excitation component voltage saturation amount output from the above-mentioned excitation component voltage limiter; a second integrator for holding the excitation component voltage saturation amount; an excitation component current command corrector for outputting a torque component current command correction amount from the held excitation component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a fifth subtracter for subtracting the torque component current command correction amount from a torque component current command and outputting a torque component current correction command, so that to operate the AC motor in an area wherein the speed largely exceeds the rated speed, occurrence of voltage saturation can also be suppressed and it is possible to perform stable control.

[0045] In a torque component current limiter for limiting a torque component current command output from a speed controller for performing proportional integration control of speed deviation between a speed command and the rotation speed of the AC motor so that the torque component current command becomes equal to or less than a predetermined value, the limit value for limiting the torque component current command is varied in response to the rotation speed of the above-mentioned AC motor, so that the magnitude of the excitation component voltage saturation amount can be lessened to some extent and it is made possible to improve the stability of control of the AC motor.

[0046] According to the invention, there is provided a speed control apparatus of an AC motor having current controllers for performing proportional integration control of an excitation component current and a torque component current of two components on rotating Cartesian two-axis coordinates into which a current of the AC motor is separated, wherein torque component current controller for performing proportional integration control of the torque component current is configured so as to continue calculation of an internal integrator even if torque component voltage component becomes saturated, the speed control apparatus comprising a torque component voltage limiter for limiting the torque component voltage component output from the torque component current controller for performing proportional integration control of the torque component current so that the torque component voltage component becomes equal to or less than a predetermined value; a first subtracter for finding a torque component voltage saturation amount from the torque component voltage component output from the above-mentioned torque component current controller and a torque component voltage command output from the above-mentioned torque component voltage limiter; a magnetic flux command corrector for outputting a magnetic flux command correction amount from the torque component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a second subtracter for subtracting the magnetic flux command correction amount from a magnetic flux command and outputting a magnetic flux correction command, so that occurrence of voltage saturation can be suppressed according to the simple configuration.

[0047] Also, there is provided a speed control apparatus of an AC motor having current controllers for performing proportional integration control of an excitation component current and a torque component current of two components on rotating Cartesian two-axis coordinates into which a current of the AC motor is separated, wherein torque component current controller for performing proportional integration control of the torque component current is configured so as to continue calculation of an internal integrator even if torque component voltage component becomes saturated, the speed control apparatus comprising a torque component voltage limiter for limiting a torque component voltage component output from torque component current controller for performing proportional integration control of the torque component current so that the torque component voltage component becomes equal to or less than a predetermined value; a first subtracter for finding a torque component voltage saturation amount from the torque component voltage component output from the above-mentioned torque component current controller and a torque component voltage command output from the above-mentioned torque component voltage limiter; an excitation component current command corrector for outputting an excitation component current command correction amount from the torque component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a third subtracter for subtracting the excitation component current command correction amount from an excitation component current command and outputting an excitation component current command correction command, so that occurrence of voltage saturation can be suppressed according to the simple configuration.

[0048] Rotation speed of the above-mentioned AC motor is input to a magnetic flux command generation section for generating a magnetic flux command and a magnetic flux command is generated in response to the rotation speed of the above-mentioned AC motor, so that the magnitude of the torque component voltage saturation amount can be lessened to some extent and it is made possible to improve the stability of control of the AC motor according to the simple configuration.

[0049] Rotation speed of the above-mentioned AC motor is input to an excitation component current command generation section for generating an excitation component current command and an excitation component current command is generated in response to the rotation speed of the above-mentioned AC motor, so that the magnitude of the torque component voltage saturation amount can be lessened to some extent and it is made possible to improve the stability of control of the AC motor according to the simple configuration.

[0050] Excitation component current controller for performing proportional integration control of the excitation component current is configured so as to continue calculation of an internal integrator even if excitation component voltage component becomes saturated, and the speed control apparatus comprises an excitation component voltage limiter for limiting the excitation component voltage component output from the excitation component current controller for performing proportional integration control of the excitation component current so that the excitation component voltage component becomes equal to or less than a predetermined value; a fourth subtracter for finding the excitation component voltage component output from the above-mentioned excitation component current controller and an excitation component voltage saturation amount output from the above-mentioned excitation component voltage limiter; an excitation component current command corrector for outputting a torque component current command correction amount from the excitation component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a fifth subtracter for subtracting the torque component current command correction amount from a torque component current command and outputting a torque component current correction command, so that to operate the AC motor in an area wherein the speed largely exceeds the rated speed, occurrence of voltage saturation can also be suppressed and it is possible to perform stable control according to the simple configuration.

[0051] In a torque component current limiter for limiting a torque component current command output from a speed controller for performing proportional integration control of speed deviation between a speed command and the rotation speed of the AC motor so that the torque component current command becomes equal to or less than a predetermined value, the limit value for limiting the torque component current command is varied in response to the rotation speed of the above-mentioned AC motor, so that the magnitude of the excitation component voltage saturation amount can be lessened to some extent and it is made possible to improve the stability of control of the AC motor according to the simple configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

[0052]FIG. 1 is a drawing to show the configuration of a speed control apparatus of an induction motor according to a first embodiment of the invention.

[0053]FIG. 2 is a graph plotting d-axis voltage component V_(d)′ and q-axis voltage component V_(q)′ based on expressions (4) and (5) for finding terminal-to-terminal voltage in a stationary state in the induction motor described above.

[0054]FIG. 3 is a drawing to show the configuration of a magnetic flux command corrector 3 a, 3 b in the speed control apparatus of the induction motor according to the first embodiment of the invention.

[0055]FIG. 4 is a drawing to show the configuration of a speed control apparatus of a permanent-magnet motor according to a second embodiment of the invention.

[0056]FIG. 5 is a drawing to show the configuration of a d-axis current command corrector 5 a, 5 b according to the second embodiment of the invention.

[0057]FIG. 6 is a drawing to show the configuration of a speed control apparatus of an induction motor according to a third embodiment of the invention.

[0058]FIG. 7 is a drawing to show the configuration of a speed control apparatus of a permanent magnet motor according to the third embodiment of the invention.

[0059]FIG. 8 is a drawing to show the configuration of a speed control apparatus of an induction motor according to a fourth embodiment of the invention.

[0060]FIG. 9 is a graph plotting d-axis voltage component V_(d)′ and the q-axis voltage component V_(q)′ based on expressions (4) and (5) for finding terminal-to-terminal voltage in a stationary state in the induction motor.

[0061]FIG. 10 is a drawing to show the configuration of a q-axis current command corrector 13 a, 13 b according to the fourth embodiment of the invention.

[0062]FIG. 11 is a drawing to show the configuration of a speed control apparatus of a permanent-magnet motor according to the fourth embodiment of the invention.

[0063]FIG. 12 is a drawing to show the configuration of a speed control apparatus of an induction motor according to a fifth embodiment of the invention.

[0064]FIG. 13 is a drawing to show the configuration of a speed control apparatus of an induction motor according to a sixth embodiment of the invention.

[0065]FIG. 14 is a drawing to show the configuration of a PI controller of a d-axis current controller 15, a q-axis current controller 16, etc., in FIG. 13.

[0066]FIG. 15 is a drawing to show the configuration of a speed control apparatus of a permanent-magnet motor according to the sixth embodiment of the invention.

[0067]FIG. 16 is a drawing to show the configuration of a speed control apparatus of an induction motor in a related art.

[0068]FIG. 17 is a drawing to show the configuration of a PI controller of a magnetic flux controller 41, a speed controller 43, a d-axis current controller 45 b, a q-axis current controller 47 b, etc., in FIG. 16.

[0069]FIG. 18 is a graph plotting d-axis voltage component V_(d)′ and q-axis voltage component V_(q)′ based on expressions for finding terminal-to-terminal voltage in a stationary state in the induction motor described later.

[0070]FIG. 19 is a graph to show the limit values of the q-axis current limiter relative to rotation speed ω_(r).

[0071]FIG. 20 is a graph to show the maximum allowable values of magnetic flux command φ_(2d)* that can be arbitrarily output from a magnetic flux command generation section relative to the rotation speed ω_(r).

[0072]FIG. 2 is a drawing to show the configuration of a current controller 45, 47 in the speed control apparatus of the induction motor according to the first embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

[0073]FIG. 1 is a drawing to show the configuration of a speed control apparatus of an induction motor according to a first embodiment of the invention. In the figure, numerals 31 to 39, 40 to 44, 45 b, 46, 48, 51, 52, 53 b, and 55 are similar to those in FIG. 16 and will not be discussed again. Numeral 1 denotes a first subtracter for outputting q-axis voltage saturation amount ΔV_(q) from q-axis voltage component V_(q)′ and q-axis voltage command V_(q)*, numeral 2 denotes an integrator for holding the q-axis voltage saturation amount ΔV_(q) and outputting held q-axis voltage saturation amount ΔV_(q)′, numeral 3 a denotes a magnetic flux command corrector for outputting magnetic flux command correction amount Δφ_(2d) from the held q-axis voltage saturation amount ΔV_(q)′ and rotation angular speed ω of dq-axis coordinates, and numeral 4 denotes a second subtracter for outputting magnetic flux correction command φ_(2d)*_(cmd) resulting from subtracting the magnetic flux command correction amount Δφ_(2d) from magnetic flux command φ_(2d)*.

[0074] Numeral 47 a denotes a q-axis current controller for controlling PI so that current deviation e_(iq) becomes 0 and outputting the q-axis voltage component V_(q)′, and numeral 54 a denotes a q-axis voltage limiter for limiting the q-axis voltage component V_(q)′ within a predetermined range and outputting the q-axis voltage command V_(q)*.

[0075]FIG. 2 is a graph plotting d-axis voltage component V_(d)′ and the q-axis voltage component V_(q)′ based on expressions (4) and (5) for finding terminal-to-terminal voltage in a stationary state in the induction motor described above; (a) indicates a graph of d-axis voltage command V_(d)* before being corrected according to the first embodiment, (b) indicates a graph of q-axis voltage command V_(q)* before being corrected according to the first embodiment, and (c) indicates a graph of q-axis voltage command V_(q)* after being corrected according to the first embodiment.

[0076]FIG. 3 is a drawing to show the configuration of the magnetic flux command corrector 3 a in the speed control apparatus of the induction motor according to the first embodiment of the invention. In the figure, numeral 21 denotes a divider for dividing the held q-axis voltage saturation amount ΔV_(q)′ by the rotation angular speed ω of dq-axis coordinates, and numeral 22 denotes a coefficient unit for inputting output of the divider 21 and outputting the magnetic flux command correction amount Δφ_(2d). However, in a magnetic flux command corrector 3 b described later, divider 21 divides the q-axis voltage saturation amount ΔV_(q) by the rotation angular speed ω of dq-axis coordinates.

[0077] The operation of the speed control apparatus of the induction motor according to the first embodiment will be discussed with FIGS. 1 to 3, FIG. 19, and FIG. 20. When voltage saturation does not occur, the speed control apparatus operates in a similar manner to that in the related art and the operation of the speed control apparatus will not be discussed again.

[0078] The terminal-to-terminal voltage of the induction motor in a stationary state is given according to expressions (4) and (5), as described above in the related art example.

[0079] A q-axis current limiter 52 is a fixed limiter with a limit value indicated by FIG. 19 (a). Assuming that a q-axis current i_(lq) flows as much as the limit value, V_(d)′ becomes the graph of FIG. 2 (a) according to expression (4). The maximum allowable value of φ_(2d)* that can be arbitrarily output from a magnetic flux command generation section 55 is indicated by FIG. 20 (b). Assuming that magnetic flux φ_(2d) takes the same value as the maximum allowable value, V_(q)′ becomes the graph of FIG. 2 (b) according to expression (5).

[0080] To operate the induction motor in an area wherein the speed is about twice the rated speed (rotation speed ω_(base)), the d-axis voltage component V_(d)′ does not exceed output limit value ±V_(max) as indicated by FIG. 2 (a). However, to operate the induction motor in an area wherein the speed is equal to or higher than the rotation speed ω_(base), the q-axis voltage component V_(q)′ exceeds output limit value ±V_(max) and voltage saturation occurs. If voltage saturation occurs, the q-axis voltage component V_(q)′ is limited to ±V_(max) by the q-axis voltage limiter 54 a. The input/output value of the q-axis voltage limiter 54 a is passed through the subtracter 1, whereby deviation (which will be hereinafter referred to as q-axis voltage saturation amount ΔV_(q)) can be found. The q-axis voltage saturation amount ΔV_(q) is a parameter indicating how much voltage is saturated, and corresponds to the V_(q)′ difference indicated by FIGS. 2 (b) and (c).

[0081] In expression (5), L₁ and M are parameters of induction motor and are fixed and the speed ω needs to be made as commanded because of the speed control apparatus and cannot be corrected. Thus, it is seen that when voltage saturation occurs due to the q-axis voltage component V_(q)′, the magnetic flux φ_(2d) must be corrected to a lower value to suppress V_(q)′. That is, the correction amount Δφ_(2d) to the magnetic flux is found from the held q-axis voltage saturation amount ΔV_(q)′ and the magnetic flux is corrected to a lower value based on the correction amount, whereby voltage saturation is eliminated.

[0082] The relationship between the held q-axis voltage saturation amount ΔV_(q)′ and the magnetic flux command correction amount Δφ_(2d) to eliminate voltage saturation is represented by expression (6) similar to expression (5).

ΔV _(q)′=(L ₁ /M)·ω·Δφ_(2d)  (6)

[0083] Further, if expression (6) is deformed with respect to the magnetic flux command correction amount Δφ_(2d), it results in expression (7).

Δφ_(2d)=(M/L ₁)·ΔV _(q)′/ω  (7)

[0084] Expression (7) becomes an expression for finding the correction amount Δφ_(2d) to the magnetic flux from the held q-axis voltage saturation amount ΔV_(q)′ and corresponds to the magnetic flux command corrector 3 a in FIG. 1 and a specific block diagram thereof is shown in FIG. 3.

[0085] The magnetic flux command correction amount Δφ_(2d) obtained as mentioned above is input to the subtracter 4 and the magnetic flux command φ_(2d)* is corrected to a lower value of the magnetic flux correction command φ_(2d)*_(cmd). According to the correction, the graph of the q-axis voltage component V_(q)′ plotted based on the expression of the terminal-to-terminal voltage becomes FIG. 2 (c) and occurrence of voltage saturation of the q-axis component can be suppressed.

[0086] In the first embodiment, if q-axis voltage saturation occurs, the degree of the voltage saturation is detected as the q-axis voltage saturation amount, the optimum magnetic flux command correction amount for eliminating the voltage saturation is determined in response to the q-axis voltage saturation amount, and the magnetic flux command is corrected in a feedback manner.

[0087] When the correction amount is determined, the speed of the motor is considered. Thus, if the speed changes rapidly, etc., the optimum correction amount can always be obtained and it is possible to suppress occurrence of voltage saturation.

[0088] Stable control can be performed independently of change in the conditions of the load and the bus voltage, and the capabilities of the PWM inverter can always be exploited at the maximum, so that it is made possible to increase output torque, etc.

[0089] The example of the induction motor has been described as the AC motor, but similar means can be used not only for the induction motor, but also for a synchronous motor for which magnetic flux control can be performed, needless to say.

Second Embodiment

[0090]FIG. 4 is a drawing to show the configuration of a speed control apparatus of a permanent-magnet motor according to a second embodiment of the invention. In the figure, numerals 1, 2, 32 to 34, 38, 39, 42 to 44, 45 b, 47 a, 48, 52, 53 b, and 54 a are similar to those in FIG. 1 and will not be discussed again.

[0091] Numeral 5 a denotes a d-axis current command corrector for inputting held q-axis voltage saturation amount ΔV_(q)′ and rotation angular speed ω of dq-axis coordinates and outputting d-axis current command correction amount Δi_(ld), and numeral 6 denotes a third subtracter for outputting d-axis current correction command i_(ld)*_(cmd) corrected by subtracting the d-axis current command correction amount Δi_(ld) from d-axis current command i_(ld)*. Numeral 56 denotes a permanent-magnet motor, numeral 57 denotes a d-axis current command generation section for outputting an arbitrary d-axis current command, and numeral 58 denotes a coefficient unit for calculating coordinate rotation angular speed.

[0092] In the first embodiment, the example of the speed control apparatus for controlling the induction motor has been shown; the second embodiment relates to the speed control apparatus for controlling a permanent-magnet motor as an AC motor.

[0093] In FIG. 4, as compared with FIG. 1 showing the configuration of the speed control apparatus for controlling the induction motor, as the AC motor to be controlled, the induction motor 31 is replaced with a permanent-magnet motor 56, the magnetic flux command generation section 55, the subtracter 4, the magnetic flux command corrector 3 a, the secondary magnetic flux calculator 35, the slip frequency calculator 36, the coordinate rotation angular speed calculator 37, the subtracter 40, the magnetic flux controller 41, and the current limiter 51 are deleted, and the d-axis current command generation section 57 for outputting an arbitrary d-axis current command, the coefficient unit 58 for calculating coordinate rotation angular speed, and the subtracter 6 are newly added. The speed control apparatus for controlling the permanent-magnet motor differs from the speed control apparatus for controlling the induction motor slightly in basic configuration, but they perform the same basic operation and also involve the same problem to be solved.

[0094]FIG. 5 is a drawing to show the configuration of the d-axis current command corrector 5 a according to the second embodiment of the invention. In the figure, numeral 23 denotes a divider for dividing the held q-axis voltage saturation amount ΔV_(q)′ by the rotation angular speed ω of dq-axis coordinates, and numeral 24 denotes a coefficient unit for inputting output of the divider 23 and outputting the d-axis current command correction amount Δi_(ld). However, in a d-axis current command corrector 5 b described later, divider 23 divides the q-axis voltage saturation amount ΔV_(q) by the rotation angular speed ω of dq-axis coordinates.

[0095] As for the permanent-magnet motor, the d-axis voltage component V_(d)′ and the q-axis voltage component V_(q)′ in a stationary state are given according to the following expressions (8) and (9):

V _(d) ′=R ₁ ·i _(1d) −L _(q) ·ω·i _(1q)  (8)

V _(q) ′=R ₁ ·i _(1q)+ω(L _(d) ·i _(1d)+φ_(f))  (9)

[0096] where R_(l) denotes primary resistance of the permanent-magnet motor 56, L_(d) denotes d-axis component inductance, L_(q) denotes q-axis component inductance, and φ_(f) denotes the maximum value of flux linkage produced by the permanent magnet.

[0097] To operate the permanent-magnet motor at the rated speed or more, each second term component becomes very larger than the first term component and thus expression (8) can be approximated by expression (10) and expression (9) can be approximated by expression (11):

V _(d) ′=−L _(q) ·ω·i _(1q)  (10)

V _(q)′=ω(L _(q) ·i _(1d)+φ_(f))  (11)

[0098] The operation of the speed control apparatus according to the second embodiment will be discussed with FIGS. 4 and 5. When voltage saturation does not occur, the speed control apparatus operates in a similar manner to that in the related art and the operation of the speed control apparatus will not be discussed again.

[0099] In the first embodiment, when voltage saturation occurs due to the q-axis voltage component V_(q)′, a correction is made to the magnetic flux command to eliminate the voltage saturation; in the second embodiment, the AC motor comprising no magnetic flux control system, the permanent-magnet motor, is to be controlled and thus the correction method is as follows:

[0100] If voltage saturation occurs, the q-axis voltage component V_(q)′ is limited to ±V_(max) by a q-axis voltage limiter 54 a. The input/output value of the q-axis voltage limiter 54 a is passed through a subtracter 1, whereby deviation (which will be hereinafter referred to as q-axis voltage saturation amount ΔV_(q)) can be found. The q-axis voltage saturation amount ΔV_(q) is a parameter indicating how much voltage is saturated.

[0101] In expression (11), L_(d) and φ_(f) are parameters of permanent-magnet motor and are fixed and the speed ω needs to be made as commanded because of the speed control apparatus and cannot be corrected. Thus, it is seen that when voltage saturation occurs due to the q-axis voltage component V_(q)′, d-axis current i_(ld) must be corrected to a lower value to suppress V_(q)′. That is, the correction amount Δi_(ld) to the d-axis current is found from the held q-axis voltage saturation amount ΔV_(q)′ and the d-axis current is corrected to a lower value based on the correction amount, whereby voltage saturation is eliminated.

[0102] The relationship between the held q-axis voltage saturation amount ΔV_(q)′ and the d-axis current command correction amount Δi_(ld) to eliminate voltage saturation can be thought according to expression (12).

ΔV _(q) ′=ω·L _(d) ·Δi _(ld)  (12)

[0103] If expression (12) is deformed with respect to the d-axis current command correction amount Δi_(ld), expression (13) is obtained.

Δi _(ld) =ΔV _(q)′/(ω·L _(d))  (13)

[0104] Expression (13) becomes an expression for deriving the correction amount Δi_(ld) to the d-axis current from the held q-axis voltage saturation amount ΔV_(q)′ and corresponds to the d-axis current command corrector 5 a in FIG. 4 and a specific block diagram thereof is shown in FIG. 5.

[0105] The d-axis current command correction amount Δi_(ld) thus obtained is input to the subtracter 6 and d-axis current command i_(ld)* is corrected to a lower value of the d-axis current correction command i_(ld)*_(cmd). According to the correction, occurrence of voltage saturation of the q-axis component can be suppressed.

[0106] As described above, according to the second embodiment, in the AC motor comprising no magnetic flux control system, occurrence of voltage saturation can also be suppressed if the speed rapidly changes as in the first embodiment, stable control can be performed independently of change in the conditions of load and bus voltage, and the capabilities of the PWM inverter can always be exploited at the maximum, so that it is made possible to increase output torque, etc.

[0107] Similar means can be used not only for the permanent-magnet motor, but also for the induction motor comprising no magnetic flux control system, needless to say. The permanent-magnet motors include an SPM motor having no silent-pole property wherein L_(d)=L_(q) and an IPM motor having silent-pole property wherein L_(d)<L_(q), but in the invention, the technique can be applied to any permanent-magnet motors regardless of the presence or absence of silent-pole property.

Third Embodiment

[0108]FIG. 6 is a drawing to show the configuration of a speed control apparatus of an induction motor according to a fourth embodiment of the invention. In the figure, numerals 1, 2, 3 a, 4, 31 to 39, 40 to 44, 45 b, 46, 47 a, 48, 51, 52, 53 b, and 54 a are similar to those in FIG. 1 and will not be discussed again. Numeral 7 denotes a magnetic flux command generation section for inputting rotation speed ω_(r) of an induction motor 31 and outputting magnetic flux command φ_(2d)* of the induction motor in response to the rotation speed ω_(r).

[0109] The operation of the speed control apparatus of an AC motor according to the third embodiment will be discussed with FIGS. 6 and 2.

[0110] In the first embodiment, the example is shown wherein if q-axis voltage saturation occurs, the magnetic flux command correction amount Δφ_(2d) found from the held q-axis voltage saturation amount ΔV_(q)′ and the rotation angular speed ω of dq-axis coordinates is subtracted from the magnetic flux command φ_(2d)* output from the magnetic flux commands generation section 55 to generate the magnetic flux correction command φ_(2d)*_(cmd). To suppress occurrence of voltage saturation, the magnetic flux correction command φ_(2d)*_(cmd) may be decreased in response to an increase in the rotation speed ω_(r).

[0111] The magnetic flux command generation section 55 generally outputs a constant value (magnetic flux command φ_(2d)*). Thus, when the rotation speed ω_(r) increases, unless the magnetic flux command correction amount Δφ_(2d) is increased, it becomes impossible to suppress occurrence of voltage saturation. As shown in FIG. 2, as the rotation speed ω_(r) increases, the q-axis voltage saturation amount ΔV_(q) grows. However, to perform stable control, it is not much preferred that the fed-back correction amount becomes too large.

[0112] In the third embodiment, the rotation speed ω_(r) is input to the magnetic flux command generation section 9 and the magnetic flux command φ_(2d)* is varied in response to the rotation speed ω_(r). For example, the magnetic flux command φ_(2d)* is varied in such a manner that the magnetic flux command φ_(2d)* is weakened in inverse proportion to an increase in the rotation speed ω_(r).

[0113] The magnetic flux command φ_(2d)* output from the magnetic flux command generation section 9 is changed in response to an increase in the rotation speed ω_(r), whereby the q-axis voltage saturation amount ΔV_(q) can be lessened and the magnetic flux command correction amount Δφ_(2d) fed back as the correction amount can be suppressed.

[0114] As described above, according to the third embodiment, the rotation speed ω_(r) is input to the magnetic flux command generation section 7 and the magnetic flux command φ_(2d)* to be output is varied accordingly, so that the magnitude of the magnetic flux command correction amount, Δφ_(2d) fed back can be lessened to some extent and it is made possible to improve the stability of control of the AC motor.

[0115]FIG. 7 is a drawing to show the configuration of a speed control apparatus of a permanent-magnet motor according to the third embodiment of the invention. In FIG. 6, the example is shown wherein the magnetic flux command generation section 55 for outputting a constant value (magnetic flux command φ_(2d)*) in the speed control apparatus of the induction motor in the first embodiment is replaced with the magnetic flux command generation section 7 for varying the magnetic flux command φ_(2d)* in response to the rotation speed ω_(r). In FIG. 7, the d-axis current command generation section 57 for outputting an arbitrary d-axis current command i_(ld)* in the second embodiment is replaced with a d-axis current command generation section 8 for varying d-axis current command i_(ld)* in response to the rotation speed ω_(r).

[0116] In control of the permanent-magnet motor, the magnitude of d-axis current command correction amount Δi_(ld) fed back can also be lessened to some extent and it is made possible to improve the stability as with the induction motor.

Fourth Embodiment

[0117]FIG. 8 is a drawing to show the configuration of a speed control apparatus of an induction motor according to a fourth embodiment of the invention. In the figure, numerals 1, 2, 3 a, 4, 31 to 39, 40 to 44, 46, 47 a, 48, 51, 52, 54 a, and 55 are similar to those in FIG. 1 and will not be discussed again.

[0118] Numeral 11 denotes a fourth subtracter for outputting d-axis voltage saturation amount ΔV_(d) from d-axis voltage component V_(d)′ and d-axis voltage command V_(d)*, numeral 12 denotes an integrator for holding the d-axis voltage saturation amount ΔV_(d) and outputting held d-axis voltage saturation amount ΔV_(d)′, numeral 13 a denotes a q-axis current command corrector for inputting the held d-axis voltage saturation amount ΔV_(d)′ and rotation angular speed ω of dq-axis coordinates and outputting q-axis current command correction amount Δi_(lq), and numeral 14 denotes a fifth subtracter for outputting q-axis current correction command i_(lj)*_(cmd) corrected by subtracting the q-axis current command correction amount Δi_(lq) from q-axis current command i_(lq)*. Numeral 45 a denotes a d-axis current controller for controlling PI so that current deviation e_(id) becomes 0 and outputting the d-axis voltage component V_(d)′, and numeral 53 a denotes a d-axis voltage limiter for limiting the d-axis voltage component V_(d)′ within a predetermined range and outputting the d-axis voltage command V_(d)*.

[0119]FIG. 9 is a graph plotting d-axis voltage component V_(d)′ and the q-axis voltage component V_(q)′ based on expressions (4) and (5) for finding terminal-to-terminal voltage in a stationary state in the induction motor; (a) indicates a graph of d-axis voltage component V_(d)′ before being corrected according to the fourth embodiment, (b) indicates a graph of q-axis voltage component V_(q)′ before being corrected according to the fourth embodiment, (c) indicates a graph of q-axis voltage component V_(q)′ after being corrected according to the fourth embodiment, and (d) indicates a graph of d-axis voltage component V_(d)′ after being corrected according to the fourth embodiment.

[0120]FIG. 10 is a drawing to show the configuration of the q-axis current command corrector 13 a according to the fourth embodiment of the invention. In the figure, numeral 25 denotes a divider for dividing the held d-axis voltage saturation amount ΔV_(d)′ by the rotation angular speed ω of dq-axis coordinates, and numeral 26 denotes a coefficient unit for inputting output of the divider 25 and outputting the q-axis current command correction amount Δi_(lq). However, in a magnetic flux command corrector 13 b described later, divider 25 divides the d-axis voltage saturation amount ΔV_(d) by the rotation angular speed ω of dq-axis coordinates.

[0121] In the first embodiment to the third embodiment, the example wherein the motor is operated in an area wherein the speed is about twice the rated speed (rotation speed ω_(base)) is shown. The fourth embodiment makes it possible to cope with the case where a motor is operated in an area wherein the speed largely exceeds the rated speed.

[0122] The operation of the speed control apparatus of the induction motor according to the fourth embodiment will be discussed with FIGS. 8 to 10, FIG. 19, and FIG. 20. When voltage saturation does not occur, the speed control apparatus operates in a similar manner to that in the related art and the operation of the speed control apparatus will not be discussed again.

[0123] The terminal-to-terminal voltage of the induction motor in a stationary state is given according to expressions (4) and (5), as described in the related art. A q-axis current limiter 52 is a fixed limiter and its q-axis current limit value is indicated by FIG. 19 (a). Assuming that a q-axis current i_(lq) flows as much as the limit value, V_(d)′ becomes the graph of FIG. 9 (a) according to expression (4). The maximum allowable value of φ_(2d)* that can be arbitrarily output from a magnetic flux command generation section 55 is indicated by FIG. 20 (b). Assuming that magnetic flux φ_(2d) takes the same value as the maximum allowable value, V_(q)′ becomes the graph of FIG. 9 (b) according to expression (5).

[0124] As shown in FIGS. 9 (a) and (b), to operate the induction motor in an area wherein the speed largely exceeds the rated speed, the q-axis voltage component V_(q)′ exceeds output limit value ±V_(max) and voltage saturation occurs in an area wherein the speed is equal to or higher than the rotation speed ω_(base), and further the d-axis voltage component V_(d)′ also exceeds output limit value ±V_(max) and voltage saturation occurs in a high-speed area wherein the speed is equal to or higher than rotation speed ω_(base2). Here, if voltage saturation of the q-axis voltage component V_(q)′ occurs in an area wherein the speed is equal to or higher than the rotation speed ω_(base) (however, less than the rotation speed ω_(base2)), the speed control apparatus operates in a similar manner to that of the speed control apparatus of the AC motor shown above in each of the first embodiment to the third embodiment, and the operation of the speed control apparatus will not be discussed again. The q-axis voltage component V_(q)′ plotted based on the expression of the terminal-to-terminal voltage is indicated by the graph of FIG. 8 (c) and occurrence of voltage saturation of the q-axis component can be suppressed.

[0125] Further, if voltage saturation of the d-axis voltage command V_(d)* occurs in an area wherein the speed is equal to or higher than the rotation speed ω_(base2), the d-axis voltage component V_(d)′ is limited to ±V_(max) by the d-axis voltage limiter 53 a. The input/output value of the d-axis voltage limiter 53 a is passed through a subtracter 1, whereby deviation (which will be hereinafter referred to as d-axis voltage saturation amount ΔV_(d)) can be found. The d-axis voltage saturation amount ΔV_(d) is a parameter indicating, how much voltage is saturated, and corresponds to the V_(d)′ difference indicated by FIGS. 9 (d) and (a).

[0126] According to expression (4), L₁ and σ are parameters of induction motor and are fixed and the speed ω needs to be made as commanded because of the speed control apparatus and cannot be corrected. Thus, it is seen that when voltage saturation occurs due to the d-axis voltage component V_(d)′, the q-axis current i_(1q) must be corrected to a lower value to suppress V_(d)′. That is, the correction amount Δi_(lq) to the q-axis current is found from the held d-axis voltage saturation amount ΔV_(d)′ and the q-axis current is corrected to a lower value based on the correction amount, whereby voltage saturation is eliminated.

[0127] The relationship between the held d-axis voltage saturation amount ΔV_(d)′ and the q-axis current command correction amount Δi_(lq) to eliminate voltage saturation is given according to expression (14) as in expression (4).

ΔV _(d) ′=−L ₁ ·σ·ω·Δi _(lq)  (14)

[0128] If expression (14) is deformed with respect to the q-axis current command correction amount Δi_(lq), it results in expression (15).

Δi _(lq) =−ΔV _(d)′/(L ₁·σ·ω)  (15)

[0129] Expression (15) becomes an expression for deriving the correction amount Δi_(lq) to the q-axis current from the held d-axis voltage saturation amount ΔV_(d)′ and corresponds to the q-axis current command corrector 13 a in FIG. 8 and a specific block diagram thereof is shown in FIG. 10.

[0130] The q-axis current command correction amount Δi_(lq) thus obtained is input to the subtracter 8 and the q-axis current command i_(lq)* is corrected to a lower value of the q-axis current correction command i_(lq)*_(cmd). According to the correction, the graph of the d-axis voltage component V_(d)′ plotted based on the theoretical expression of the terminal-to-terminal voltage becomes FIG. 9 (d) and occurrence of voltage saturation of the d-axis component can be suppressed.

[0131] As described above, according to the fourth embodiment, if d-axis voltage saturation occurs, the degree of the voltage saturation is detected as the d-axis voltage saturation amount, and the optimum q-axis current command correction amount for eliminating the voltage saturation is determined in response to the d-axis voltage saturation amount and is fedback to correct the q-axis current command.

[0132] When the correction amount is determined, the speed of the motor is considered. Thus, if the speed changes rapidly, etc., the optimum correction amount can always be obtained and it is possible to suppress occurrence of voltage saturation.

[0133] To operate the AC motor in an area wherein the speed largely exceeds the rated speed, stable control can also be performed independently of change in the conditions of load and bus voltage, and the capabilities of the PWM inverter can always be exploited at the maximum, so that it is made possible to increase output torque, etc.

[0134]FIG. 11 is a drawing to show the configuration of a speed control apparatus of a permanent-magnet motor according to the fourth embodiment of the invention. In the figure, numerals 1, 2, 5 a, 6, 32 to 34, 38, 39, 42 to 44, 46, 47 a, 48, 54 a, and 56 to 58 are similar to those in FIG. 4 shown in the second embodiment and will not be discussed again. Numeral 11 denotes a fourth subtracter for outputting d-axis voltage saturation amount ΔV_(d) from d-axis voltage component V_(d)′ and d-axis voltage command V_(d)*, numeral 12 denotes an integrator for holding the d-axis voltage saturation amount ΔV_(d) and outputting held d-axis voltage saturation amount ΔV_(d)′, numeral 13 a denotes a q-axis current command corrector for inputting the held d-axis voltage saturation amount ΔV_(d)′ and rotation angular speed ω of dq-axis coordinates and outputting q-axis current command correction amount Δi_(lq), and numeral 14 denotes a fifth subtracter for outputting q-axis current correction command i_(lq)*_(cmd) corrected by subtracting the q-axis current command correction amount Δi_(lq) from q-axis current command i_(lq)*. Numeral 45 a denotes a d-axis current controller for controlling PI so that current deviation e_(id) becomes 0 and outputting the d-axis voltage component V_(d)′, and numeral 53 a denotes a d-axis voltage limiter for limiting the d-axis voltage component V_(d)′ within a predetermined range and outputting the d-axis voltage command V_(d)*.

[0135]FIG. 11 shows an example of using the fourth embodiment of the invention for the speed control apparatus of the permanent-magnet motor, and the operation of the speed control apparatus is similar to that of the speed control apparatus of the induction motor in FIG. 8 and will not be discussed again.

Fifth Embodiment

[0136]FIG. 12 is a drawing to show the configuration of a speed control apparatus of an induction motor according to a fifth embodiment of the invention. In the figure, numerals 1, 2, 3 a, 4, 11, 12, 13 a, 14, 31 to 39, 40 to 44, 45 a, 46, 47 a, 48, 51, 53 a, 54 a, and 55 are similar to those in FIG. 8 and will not be discussed again. Numeral 15 denotes a q-axis current limiter for inputting rotation speed ω_(r) of an induction motor 31 and varying a limit value in response to the rotation speed ω_(r).

[0137] The operation of the speed control apparatus of the AC motor according to the fifth embodiment will be discussed with FIGS. 12 and 9.

[0138] In the fourth embodiment, if d-axis voltage saturation occurs, the degree of the voltage saturation is detected as the d-axis voltage saturation amount ΔV_(d), and the optimum q-axis current command correction amount Δi_(lq) for eliminating the voltage saturation is determined in response to the d-axis voltage saturation amount and the q-axis current command correction amount Δi_(lq) is fed back to correct the q-axis current command I_(lq)*. Here, as shown in FIG. 9 for the fourth embodiment, as the rotation speed ω_(r) increases, the d-axis voltage saturation amount ΔV_(d) grows. However, to perform stable control, it is not much preferred that the fed-back correction amount becomes too large.

[0139] In the fourth embodiment, the result of subtracting the q-axis current command correction amount Δi_(lq) fed back as the correction amount from the q-axis current command i_(lq)* output from the q-axis current limiter 52 becomes the final q-axis current correction command i_(lq)*_(cmd). To suppress occurrence of voltage saturation, the q-axis current correction command i_(lq)*_(cmd) may be small for an increase in the rotation speed ω_(r).

[0140] However, the q-axis current limiter 52 in the fourth embodiment is a fixed limiter and always limits with a constant value. Thus, when the rotation speed ω_(r) increases and the q-axis current command is output fully up to the limit value, unless the q-axis current command correction amount Δi_(lq) is increased, it becomes impossible to suppress occurrence of voltage saturation.

[0141] In the fifth embodiment, the q-axis current limiter 52 of a fixed limiter in the fourth embodiment is replaced with the q-axis current limiter 15 of a variable limiter for varying the limit value in response to the rotation speed ω_(r). For example, the limit value is varied in such a manner that the limit value is weakened in inverse proportion to an increase in the rotation speed ω_(r).

[0142] The q-axis current command i_(lq)* output from the q-axis current limiter 15 is variably limited for an increase in the rotation speed ω_(r), whereby the d-axis voltage saturation amount ΔV_(d) can be lessened, and the q-axis current command correction amount Δi_(lq) fed back as the correction amount can be suppressed.

[0143] The control example of the induction motor has been shown. However, in the speed control apparatus of the permanent-magnet motor in FIG. 1, the q-axis current limiter 52 of a fixed limiter is replaced with the q-axis current limiter 15 of a variable limiter for varying the limit value in response to the rotation speed ω_(r), whereby stability can also be improved for control of the permanent magnet motor.

[0144] As described above, according to the fifth embodiment, the q-axis current limiter 15 is made a variable limiter for varying the limit value in response to the rotation speed ω_(r), so that the magnitude of the d-axis voltage saturation amount ΔV_(d) can be lessened to some extent and it is made possible to improve the stability of control of the AC motor.

[0145] It is also possible to use the fifth embodiment and the third embodiment in combination, lessen both the d-axis voltage saturation amount ΔV_(d) and the q-axis voltage saturation amount ΔV_(q) to some extent, and remarkably improve the stability of control of the AC motor.

Sixth Embodiment

[0146]FIG. 13 is a drawing to show the configuration of a speed control apparatus of an induction motor according to a sixth embodiment of the invention. In the figure, numerals 1, 4, 11, 14, 31 to 39, 40 to 44, 46, 48, 51, 52, and 55 are similar to those in FIG. 8 and will not be discussed again. Numeral 3 b denotes a magnetic flux command corrector for outputting magnetic flux command correction amount Δφ_(2d) from q-axis voltage saturation amount ΔV_(q) and rotation angular speed ω of dq-axis coordinates, numeral 13 b denotes a q-axis current command corrector for inputting d-axis voltage saturation amount ΔV_(d) and rotation angular speed ω of dq-axis coordinates and outputting q-axis current command correction amount Δi_(lq), numeral 16 denotes a d-axis current controller for controlling PI so that current deviation e_(id) becomes 0 and outputting d-axis voltage component V_(d)′, numeral 17 denotes a q-axis current controller for controlling PI so that current deviation e_(iq) becomes 0 and outputting q-axis voltage component V_(q)′, numeral 18 denotes a d-axis voltage limiter for limiting the d-axis voltage component V_(d)′ within a predetermined range and outputting d-axis voltage command V_(d)*, and numeral 19 denotes a q-axis voltage limiter for limiting the q-axis voltage component V_(q)′ within a predetermined range and outputting q-axis voltage command V_(q)*.

[0147]FIG. 14 is a drawing to show the configuration of a PI controller of a current controller 16, 17 used in the speed control apparatus of the induction motor according to the sixth embodiment of the invention. In the figure, numerals 61, 62, and 64 are similar to those in FIG. 17 of the related art example and will not be discussed again. Numeral 63 a denotes an integrator.

[0148] Letter e denotes deviation input to the PI controller and U′ denotes control input output from the PI controller. As for the d-axis current controller 16, e corresponds to the current deviation e_(id) between d-axis current command i_(ld)* and d-axis current i_(ld), and U′ corresponds to the d-axis voltage component V_(d)′. As for the q-axis current controller 17, e corresponds to current deviation e_(iq) between the q-axis current command i_(lq)* and q-axis current i_(lq), and U′ corresponds to the q-axis voltage component V_(q)′.

[0149] If the control input U′ exceeds the limit value of the d-axis voltage limiter 53 a, 53 b, the q-axis voltage limiter 54 a, 54 b, the d-axis current controller 45 a, 45 b, the q-axis current controller 47 a, 47 b used in the related art example and the first embodiment to the fifth embodiment is configured for stopping the calculation of the integrator 63 in the current controller for controlling PI and thus the integrator 12 for holding the d-axis voltage saturation amount ΔV_(d) and the integrator 2 for holding the q-axis voltage saturation amount ΔV_(q) are added. However, even if the control input U′ exceeds the limit value of the d-axis voltage limiter 18, the q-axis voltage limiter 19, the d-axis current controller 16 and the q-axis current controller 17 used in the sixth embodiment cause each a value equal to or greater than the limit value to be held in the internal integrator 63 a without stopping the calculation of the integrator 63 a in the current controller for controlling PI.

[0150] In the sixth embodiment, the d-axis current controller 45 a and the q-axis current controller 47 a in the fourth embodiment are replaced with the d-axis current controller 16 and the q-axis current controller 17, the integrator 12 for holding the d-axis voltage saturation amount ΔV_(d) and the integrator 2 for holding the q-axis voltage saturation amount ΔV_(q) are eliminated, the magnetic flux command corrector 3 a for outputting the magnetic flux command correction amount Δφ_(2d) from the q-axis voltage saturation amount ΔV_(q)′ held in the integrator 2 and the rotation angular speed ω of dq-axis coordinates is replaced with the magnetic flux command corrector 3 b for outputting the magnetic flux command correction amount Δφ_(2d) from the q-axis voltage saturation amount ΔV_(q) and the rotation angular speed ω of dq-axis coordinates, and the q-axis current command corrector 13 a for inputting the d-axis voltage saturation amount ΔV_(d)′ held in the integrator 12 and the rotation angular speed ω of dq-axis coordinates and outputting the q-axis current command correction amount Δi_(lq) is replaced with the q-axis current command corrector 13 b for inputting the d-axis voltage saturation amount ΔV_(d) and the rotation angular speed ω of dq-axis coordinates and outputting the q-axis current command correction amount Δi_(lq), whereby equal operation is performed. The operation of the speed control apparatus is similar to that of the speed control apparatus of the fourth embodiment and therefore will not be discussed again.

[0151] The example wherein the current controllers 45 a and 47 a in FIG. 8 are replaced with the current controllers 16 and 17 has been described but the current controllers 45 a and 47 a in FIG. 12 may be replaced with the current controllers 16 and 17. The current controller 47 a in FIG. 6 may be replaced with the current controller 17.

[0152] The d-axis current controller 16 and the q-axis current controller 17 of the PI controllers designed for not stopping the calculation of the integrator 63 a in the PI controller even if the control input U′ exceeds the limit value are used, so that occurrence of voltage saturation can be suppressed according to the simple configuration.

[0153]FIG. 15 is a drawing to show the configuration of a speed control apparatus of a permanent-magnet motor according to the sixth embodiment of the invention. In the figure, numerals 1, 11, 32 to 34, 38, 39, 42 to 44, 46, 48, 51, 52, and 56 to 58 are similar to those in FIG. 11 and will not be discussed again. Numeral 5 b denotes a d-axis current command corrector for inputting q-axis voltage saturation amount ΔV_(q) and rotation angular speed ω of dq-axis coordinates and outputting d-axis current command correction amount Δi_(ld), numeral 13 b denotes a q-axis current command corrector for inputting d-axis voltage saturation amount ΔV_(d) and rotation angular speed ω of dq-axis coordinates and outputting q-axis current command correction amount Δi_(lq), numeral 16 denotes a d-axis current controller for controlling PI so that current deviations e_(id) becomes 0 and outputting d-axis voltage component V_(d)′, numeral 17 denotes a q-axis current controller for controlling PI so that current deviation e_(iq) becomes 0 and outputting q-axis voltage component V_(q)′, numeral 18 denotes a d-axis voltage limiter for limiting the d-axis voltage component V_(d)′ within a predetermined range and outputting d-axis voltage command V_(d)*, and numeral 19 denotes a q-axis voltage limiter for limiting the q-axis voltage component V_(q)′ within a predetermined range and outputting q-axis voltage command V_(q)*.

[0154] In FIG. 15, the d-axis current controller 45 a and the q-axis current controller 47 a in FIG. 11 are replaced with the d-axis current controller 16 and the q-axis current controller 17 each for causing a value equal to or greater than the limit value to be held in the internal integrator 63 a without stopping the calculation of the integrator 63 a in the current controller for controlling PI even if the control input U′ exceeds the limit value of the d-axis voltage limiter 18, the q-axis voltage limiter 19, the integrator 12 for holding the d-axis voltage saturation amount ΔV_(d) and the integrator 2 for holding the q-axis voltage saturation amount ΔV_(q) are eliminated, the magnetic flux command corrector 3 a for outputting the magnetic flux command correction amount Δφ_(2d) from the q-axis voltage saturation amount ΔV_(q)′ held in the integrator 2 and the rotation angular speed ω of dq-axis coordinates is replaced with the magnetic flux command corrector 3 b for outputting the magnetic flux command correction amount Δφ_(2d) from the q-axis voltage saturation amount ΔV_(q) and the rotation angular speed ω of dq-axis coordinates, and the q-axis current command corrector 13 a for inputting the d-axis voltage saturation amount ΔV_(d)′ held in the integrator 12 and the rotation angular speed ω of dq-axis coordinates and outputting the q-axis current command correction amount Δi_(lq) is replaced with the q-axis current command corrector 13 b for inputting the d-axis voltage saturation amount ΔV_(d) and the rotation angular speed ω of dq-axis coordinates and outputting the q-axis current command correction amount Δi_(lq), whereby equal operation is performed. The operation of the speed control apparatus is similar to that of the speed control apparatus of the fourth embodiment and therefore will not be discussed again.

[0155] The example wherein the current controllers 45 a and 47 a in FIG. 11 are replaced with the current controllers 16 and 17 has been described, but the current controller 47 a in FIG. 4 may be replaced with the current controller 17.

[0156] The d-axis current controller 16 and the q-axis current controller 17 of the PI controllers designed for not stopping the calculation of the integrator 63 a in the PI controller even if the control input U′ exceeds the limit value are used, so that occurrence of voltage saturation can be suppressed according to the simple configuration.

[0157] Industrial Applicability

[0158] As described above, if voltage saturation occurs in the speed control apparatus of the AC motor, the optimum correction amount for eliminating the voltage saturation is found based on the voltage saturation amount detected as the voltage saturation degree and is fed back to correct each command, so that the speed control apparatus is suited for use in application wherein high-speed operation at the rated speed or higher is performed or rapid speed change is made. 

1. A speed control apparatus of an AC motor having current controllers for performing proportional integration control of an excitation component current and a torque component current of two components on rotating Cartesian two-axis coordinates into which a current of the AC motor is separated, said speed control apparatus comprising: a torque component voltage limiter for limiting a torque component voltage component output from torque component current controller for performing proportional integration control of the torque component current so that the torque component voltage component becomes equal to or less than a predetermined value; a first subtracter for finding a torque component voltage saturation amount from the torque component voltage component output from said torque component current controller and a torque component voltage command output from said torque component voltage limiter; a first integrator for holding the torque component voltage saturation amount; a magnetic flux command corrector for outputting a magnetic flux command correction amount from the held torque component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a second subtracter for subtracting the magnetic flux command correction amount from a magnetic flux command and outputting a magnetic flux correction command.
 2. A speed control apparatus of an AC motor having current controllers for performing proportional integration control of an excitation component current and a torque component current of two components on rotating Cartesian two-axis coordinates into which a current of the AC motor is separated, said speed control apparatus comprising: a torque component voltage limiter for limiting a torque component voltage component output from torque component current controller for performing proportional integration control of the torque component current so that the torque component voltage component becomes equal to or less than a predetermined value; a first subtracter for finding a torque component voltage saturation amount from the torque component voltage component output from said torque component current controller and a torque component voltage command output from said torque component voltage limiter; a first integrator for holding the torque component voltage saturation amount; an excitation component current command corrector for outputting an excitation component current command correction amount from the held torque component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a third subtracter for subtracting the excitation component current command correction amount from an excitation component current command and outputting an excitation component current command correction command.
 3. The speed control apparatus of an AC motor as claimed in claim 1, wherein rotation speed of said AC motor is input to a magnetic flux command generation section for generating a magnetic flux command, and a magnetic flux command is generated in response to the rotation speed of said AC motor.
 4. The speed control apparatus of an AC motor as claimed in claim 2, wherein rotation speed of said AC motor is input to an excitation component current command generation section for generating an excitation component current command, and an excitation component current command is generated in response to the rotation speed of said AC motor.
 5. The speed control apparatus of an AC motor as claimed in any of claims 1 to 4 comprising: an excitation component voltage limiter for limiting an excitation component voltage component output from excitation component current controller for performing proportional integration control of the excitation component current so that the excitation component voltage component becomes equal to or less than a predetermined value; a fourth subtracter for finding the excitation component voltage component output from said excitation component current controller and an excitation component voltage saturation amount output from said excitation component voltage limiter; a second integrator for holding the excitation component voltage saturation amount; an excitation component current command corrector for outputting a torque component current command correction amount from the held excitation component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a fifth subtracter for subtracting the torque component current command correction amount from a torque component current command and outputting a torque component current correction command.
 6. The speed control apparatus of an AC motor as claimed in claim 5, wherein in a torque component current limiter for limiting a torque component current command output from a speed controller for performing proportional integration control of speed deviation between a speed command and the rotation speed of said AC motor so that the torque component current command becomes equal to or less than a predetermined value, the limit value for limiting the torque component current command is varied in response to the rotation speed of said AC motor.
 7. A speed control apparatus of an AC motor having current controllers for performing proportional integration control of an excitation component current and a torque component current of two components on rotating Cartesian two-axis coordinates into which a current of said AC motor is separated, said speed control apparatus comprising: a torque component current controller for performing proportional integration control of the torque component current is configured so as to continue calculation of an internal integrator even if torque component voltage component becomes saturated; a torque component voltage limiter for limiting the torque component voltage component output from said torque component current controller for performing proportional integration control of the torque component current so that the torque component voltage component becomes equal to or less than a predetermined value; a first subtracter for finding a torque component voltage saturation amount from the torque component voltage component output from said torque component current controller and a torque component voltage command output from said torque component voltage limiter; a magnetic flux command corrector for outputting a magnetic flux command correction amount from the torque component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a second subtracter for subtracting the magnetic flux command correction amount from a magnetic flux command and outputting a magnetic flux correction command.
 8. A speed control apparatus of an AC motor having current controllers for performing proportional integration control of an excitation component current and a torque component current of two components on rotating Cartesian two-axis coordinates into which a current of said AC motor is separated, said speed control apparatus comprising: a torque component current controller for performing proportional integration control of the torque component current is configured so as to continue calculation of an internal integrator even if torque component voltage component becomes saturated; a torque component voltage limiter for limiting a torque component voltage component output from torque component current controller for performing proportional integration control of the torque component current so that the torque component voltage component becomes equal to or less than a predetermined value; a first subtracter for finding a torque component voltage saturation amount from the torque component voltage component output from said torque component current controller and a torque component voltage command output from said torque component voltage limiter; an excitation component current command corrector for outputting an excitation component current command correction amount from the torque component voltage saturation amount and rotation angular speed of Cartesian two-axis coordinates; and a third subtracter for subtracting the excitation component current command correction amount from an excitation component current command and outputting an excitation component current command correction command.
 9. The speed control apparatus of an AC motor as claimed in claim 7, wherein rotation speed of said AC motor is input to a magnetic flux command generation section for generating a magnetic flux command, and a magnetic flux command is generated in response to the rotation speed of said AC motor.
 10. The speed control apparatus of an AC motor as claimed in claim 8, wherein rotation speed of said AC motor is input to an excitation component current command generation section for generating an excitation component current command, and an excitation component current command is generated in response to the rotation speed of said AC motor.
 11. The speed control apparatus of an AC motor as claimed in any of claims 7 to 10, said speed control apparatus comprising: an excitation component current controller for performing proportional integration control of the excitation component current is configured so as to continue calculation of an internal integrator even if excitation component voltage component becomes saturated; an excitation component voltage limiter for limiting the excitation component voltage component output from said excitation component current controller for performing proportional integration control of the excitation component current so that the excitation component voltage component becomes equal to or less than a predetermined value; a fourth subtracter for finding the excitation component voltage component output from said excitation component current controller and an excitation component voltage saturation amount output from said excitation component voltage limiter; an excitation component current command corrector for outputting a torque component current command correction amount from the excitation component voltage saturation amount and rotation angular speed of Cartesian two axis coordinates; and a fifth subtracter for subtracting the torque component current command correction amount from a torque component current command and outputting a torque component current correction command.
 12. The speed control apparatus of an AC motor as claimed in claim 11, wherein in a torque component current limiter for limiting a torque component current command output from a speed controller for performing proportional integration control of speed deviation between a speed command and the rotation speed of said AC motor so that the torque component current command becomes equal to or less than a predetermined value, the limit value for limiting the torque component current command is varied in response to the rotation speed of said AC motor. 